The elimination of hydrogen (dehydrogenation) from an alcohol gives an aldehyde. Conversely, alcohols can be prepared from aldehydes by hydrogenation (addition of hydrogen). Hydrogenation in general is a reaction carried out very frequently in industrial technology. But also specifically the hydrogenation of aldehydes is practised on an industrial scale, namely in the production of so-called oxo alcohols.
Oxo alcohols are alcohols which are produced in the course of hydroformylation (oxo reaction). During the hydroformylation, an olefin (alkene) is reacted with a synthesis gas (this is a mixture of carbon monoxide and hydrogen) to give an aldehyde. Subsequent hydrogenation gives the actual oxo alcohol. Oxo alcohols serve as intermediates for producing surfactants and/or plasticizers for plastic. Worldwide, several million tons of oxo alcohols are produced every year. Since the hydrogenation of the aldehydes obtained by the hydroformylation is a necessary step in the production of oxo alcohols, the present invention deals with a process that is of relevance on an industrial scale.
In industrial practice, oxo aldehydes are generally hydrogenated in the liquid phase on heterogeneous fixed-bed catalysts. On account of the large throughput amounts, the catalyst is attributed the decisive importance for the process since it determines the reaction rate and also the selectivity of the hydrogenation. The selection of a suitable catalyst is not trivial since the aldehydes to be hydrogenated never occur in pure form, but as a mixture of structural isomeric aldehydes which is always accompanied by a large number of troublesome accompanying components which firstly bring about secondary reactions undesired in the hydrogenation and secondly damage the hydrogenation catalyst. Since the composition of the use mixture comprising the aldehydes to be hydrogenated is determined by the upstream hydroformylation, the hydrogenation catalyst has to be precisely adapted to the particular hydroformylation.
For the hydrogenation of oxo aldehydes, catalysts that have proven useful are those which comprise a support material on which copper, chromium and nickel are applied as active components.
A corresponding catalyst is disclosed in DE19842370A1. It comprises copper and nickel, in each case in a concentration range from 0.3 to 15% by weight and chromium in a weight fraction of from 0.05% by weight to 3.5% by weight. The support material used is porous silicon dioxide or aluminium oxide.
EP1219584B1 reveals that such a catalyst also manages with the accompanying component water, which is to be expected in particular during the co-catalysed hydroformylation. Water is critical since it can have a lasting negative influence on the surface structure of the catalyst by, for example, reducing the specific surface area. For this reason, the aldehyde mixtures originating from the cobalt-catalysed hydroformylation are particularly demanding to hydrogenate.
A further development of this Ni/Cu/Cr catalyst consists in adding barium to the support material (EP2180947B1).
WO2009/146988A1 deals with the two-stage hydrogenation of oxo alcohols over two different Ni/Cu/Cr catalysts.
Although the nickel/copper/chromium catalysts have proven useful in the industrially practised hydrogenation of oxo aldehydes, there is still the need for an alternative. The reason for this is the chromium present.
According to Annex XIV of the REACH regulation, chromium-containing substances such as the catalysts described above may only be used in the European Union after authorization by the Commission. The granting of authorization is associated with great complexity and high costs; moreover, granting of authorization cannot be expected a priori. Moreover, the application procedure has to be repeated every five years.
The reason for these strict conditions is the undisputed carcinogenicity of the chromium(IV) compounds present in the catalyst. These are firstly relevant when hydrogenation catalyst has to be disposed of following deactivation and, secondly, when it is newly produced by impregnation with alkali metal chromates or alkali metal dichromates.
For health and cost reasons there is therefore a great need for a chromium-free alternative to the hydrogenation of oxo aldehydes.
Chromium-free hydrogenation catalysts are disclosed in EP1749572A1. The support material used is porous aluminium oxide and the hydrogenation-active components are nickel or cobalt. The examples reveal that a Ni/Al2O3 or a Co/Al2O3 system is suitable for the hydrogenation of oxo aldehydes; the properties and productivity of these systems, however, has not been investigated. The disadvantage of the cobalt catalysts shown in EP1749572A1 consists in the fact that they in any case have to be reduced at relatively high temperatures of 350° C. to 450° C. This usually does not take place in-situ in the reactor since the reactors for the aldehyde hydrogenation are designed only for a temperature of up to about 200° C. Consequently, the cobalt catalysts have to be reduced ex situ and then be incorporated into the hydrogenation reactor under a protective atmosphere. This is very complex. Moreover, cobalt is a comparatively expensive material.
For the nickel catalysts, approx. 200° C. could just suffice for an in situ reaction. However, EP1749572A1 mentions that the Ni/Al2O3 system favours the further reaction and the reaction must therefore be ended very quickly. That is not always easy to accomplish in industrial use. The chromium-free hydrogenation catalysts shown in EP1749572A1 have overall so many disadvantages that they are not real alternatives to the classic Ni/Cu/Cr systems.
DE3737277C2 discloses a chromium-free catalyst for the hydrogenation of aldehydes which is based on copper/zinc oxides. Potassium, nickel and/or cobalt and additionally an alkali metal are present as further hydrogenation-active metals. This system is a so-called uniform-composition catalyst which consists exclusively of the hydrogenation-active materials. Such uniform-composition catalysts are very expensive to produce and are therefore too costly for industrial use. After all of this, it has hitherto not been possible to find a chromium-free catalyst which is suitable for the hydrogenation of hydroformylation mixtures on an industrial scale and which can be produced easily.